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4.4 Amphibian Organizer — Test 1
Q1. The Spemann–Mangold organizer in amphibians is located at the:✓ Dorsal lip of the blastopore
Q2. Grafting the dorsal lip of an early gastrula onto the ventral side of a host embryo produces:✓ Two complete (conjoined) embryos
Q3. The dorsal lip of the blastopore is therefore called the:✓ Primary organizer
Q4. The amphibian organizer induces neural tissue mainly by secreting proteins that:✓ Inhibit BMP signalling
Q5. Which protein secreted by the amphibian organizer induces neural tissue by inhibiting BMP?✓ Noggin
Q6. Chordin, Noggin and Follistatin are all:✓ BMP antagonists secreted by the organizer
Q7. The default fate of amphibian ectoderm, in the absence of BMP signalling, is to become:✓ Neural tissue
Q8. The group of cells that induces the amphibian organizer is the:✓ Nieuwkoop centre
Q9. The Nieuwkoop centre in Xenopus is made of which cells?✓ Dorsal-most vegetal (endodermal) cells
Q10. The Nieuwkoop centre is marked by the nuclear accumulation of:✓ beta-catenin
Q11. The initial dorsal–ventral axis of the amphibian embryo is determined by:✓ The point of sperm entry
Q12. Cortical rotation after fertilization in amphibians leads to the dorsal accumulation of:✓ beta-catenin (stabilised dorsally)
4.4 Amphibian Organizer — Test 2
Q13. Separating an amphibian embryo into two at the two-cell stage typically gives:✓ Identical twins
Q14. Expressing a dominant-negative GSK3 in the ventral cells of an early amphibian embryo will:✓ Cause a second axis to form
Q15. In Xenopus, the Nieuwkoop centre's beta-catenin helps activate which dorsal genes?✓ siamois and twin
Q16. The Spemann–Mangold organizer is able to:✓ Initiate the movements of gastrulation and induce a second axis
Q17. Injecting Noggin mRNA into the future ventral side of a frog embryo:✓ Mimics an organizer graft and can induce a secondary axis
Q18. Noggin protein accomplishes which two functions of the organizer?✓ Inducing neural tissue and dorsalizing mesoderm
Q19. In the activin-bead experiment in Xenopus, cells nearest the bead (highest activin) express:✓ goosecoid (specifying dorsal-most structures)
Q20. In the activin gradient, higher concentrations activate goosecoid while lower concentrations activate:✓ Xbra
Q21. In the activin experiment, cells farthest from the bead (negligible activin) become, by default:✓ Blood and ventral mesoderm (e.g. blood vessels, heart)
Q22. Fibronectin in the amphibian gastrula is important because it:✓ Provides a substrate that mesodermal cells migrate along
Q23. Wnt signalling sets up a gradient of beta-catenin along the neural tube that helps to:✓ Specify the anterior–posterior regionalization of the neural tube